In case of a conventional electric vehicle or a hybrid vehicle, it is necessary to connect a plug of the electric vehicle directly to an electric charger provided at a charging station and to wait for a substantial amount of time until the batteries therein are fully charged. In addition, due to the limited capacity of the batteries, the electric vehicle may need recharging several times to make a long distance travel. Further, even if a fast-charging device is used, one hour or more may be taken to charge the battery. Even with the state-of-the-art technology, ten minutes or more may be required in charging the batteries of an electric vehicle.
Recently, a new technology capable of increasing the battery capacity and reducing the charging time of the electric vehicle has been developed to overcome these drawbacks. In accordance with this new technology, however, use of a heavier battery is required, which may adversely affect the efficiency of the vehicle and increase the manufacturing cost thereof. Further, reduction of the charging time may be achieved at the expense of the battery lifespan.
An electromagnetic induction charging system for an electric vehicle has been proposed as an alternative to the battery-type electric vehicle system; however, the former is known to suffer from the low power transfer efficiency. For instance, the PATH (Partners for Advanced Transit and Highways) team led by University of California at Berkeley has developed an electromagnetic induction charging system for an electric vehicle that allows the electric vehicle to operate with a gap of at most 7 cm between a primary coil embedded in the road and a secondary coil attached to the electric vehicle. In this connection, when the gap between the primary and the secondary coils has a shorter distance than 7 cm, it has been reported that a power transfer efficiency of about 60% can be achieved by the electromagnetic induction charging system developed by the PATH. However, when the primary and the secondary coils are spaced apart by a distance of 7 cm, the power transfer efficiency of this electromagnetic induction charging system falls below a practicable level. Further, from the realistic point of view, even the gap of 7 cm between the primary and the secondary coils is too short a distance, since a typical gap between a bottom of a vehicle and a surface of the road is about 20 to 30 cm.
Another problem associated with the prior art electromagnetic induction charging system for an electric vehicle lies in the limited freedom of lateral movement or deviation. When E-shaped magnetic cores having a lateral width of about 100 cm are used for a power supply device and a power acquisition device, respectively, and if a center of the power acquisition device attached to the electric vehicle is deviated 5 cm or more from a center of the power supply device to a lateral direction, the primary coil winding around the center portion of the magnetic core of the power supply device and the secondary coil winding around the center portion of the magnetic core of the power acquisition device will get out of alignment, thereby reducing the efficiency of power transfer by 10% or more. In order to avoid such a situation, therefore, it is necessary for a driver to carefully drive an electric vehicle along the median area of a lane on the road. Otherwise, the electric vehicle needs to be equipped with, e.g., a line tracer for controlling the lateral movement thereof.
As discussed above, if the gap between the primary and the secondary coils is reduced in an effort to attain a high power transfer efficiency, the power acquisition device may collide with an obstacle present on the road and may be damaged by the collision. Further, the power transfer efficiency between the power supply device and the power acquisition device may be greatly affected by a lateral deviation of the electric vehicle. Accordingly, there has existed a need to solve these problems which have impeded the development and commercialization of an electric vehicle which acquires power by electromagnetic induction from a power supply device embedded in a road.